c. 1600, from Latin apex "summit, peak, tip, top, extreme end;" probably related to apere "to fasten, fix," hence "the tip of anything" (one of the meanings in Latin was "small rod at the top of the flamen's cap"), from PIE *ap- "to take, reach." Proper plural is apices.
1. At the apex of the party was its central committee.
2. The apex of the flames licked the crimson sky.
3. The Apex fare is £195 return.
4. He reached the apex of power in the early 1930 s.
5. Spring was the real apex of the year; summer was bathos.